Electrostatic deposition of particles

ABSTRACT

A device for spraying particles for example onto crops comprises a barrel with a discharge nozzle and means for inducing, by the triboelectric effect or other means, electric charge into the particles. Preferably means for generating an electric field are provided near the nozzle, this field serving to propel and guide particles onto the target.

United States Patent 1 Coffee 1 Sept. 16, 1975 [54] ELECTROSTATIC DEPOSITION OF 3.133.828 5/1964 Slatkin 118/620 PARTICLES 3,195164 7/1965 Ward c 4 i v c 1 i 239/3 3.6135993 10/1971 Gourdine et a1 239/15 1 Inventor: Ronald Alan Coffee, El 2 Mount 3.745.034 7/1973 Smith et a1 1l8/308 Davis Rd., Hong Kong Hong Kong [22] Filed 1974 Primary ExaminerMervin Stein [2] 1 Applr No.: 493,964 Assn-Ian! Examiner-Steven Hawkins Anomey, Agent, or FirmEdwin E, Greigg [30] Foreign Application Priority Data Nov, 21 1973 United Kingdom 53939/73 Nov. 21, 1973 United Kingdom 53940/73 [57] ABSTRACT May 16. 1974 United Kingdom 21764/74 A device for spraying particles for example onto crops 1521 118/629; 1 18/308; 118/621; comprises a barrel with a discharge nozzle and means 239/15 for inducing, by the triboelectric effect or other 1 B05: 5/02 means, electric charge into the particles. Preferably Field of Search 118/308- means for generating an electric field are provided 1 18/629; 23 /1 3 near the nozzle, this field serving to propel and guide particles onto the target 56] References Cited UNITED STATES PATENTS Reindl .7 118/629 16 Claims, 6 Drawing Figures ELECTROSTATIC DEPOSITION OF PARTICLES This invention relates to the electrostatic deposition of particles for example onto crops.

According to one aspect the present invention provides a device for electrostatically depositing particles onto a surface at earth (or other relatively low) potential, such device comprising a casing, means for introducing particles to be deposited into the interior of the casing, a discharge barrel extending from the casing and triboelectricity inducing means movable within the casing which also propel particles from the casing through and out of the barrel.

The triboelectricity inducing means can comprise an impeller fan with an attached inlet from a container of powder.

According to another aspect the invention provides a device for electrostatically depositing particles on a surface at a lower potential such device comprising a tubular conduit, means for introducing particles into the conduit, means for driving particles along the conduit to a discharge outlet, means for inducing by triboelectricity electric charge to the particles driven along the conduit, electrically chargeable means for generating a directional electric field around or nearby said discharge outlet and means for conductively connecting said electrically chargeable means with charged particles in the conduit.

According to another aspect the invention provides a device for electrostatically depositing particles such device comprising a first duct forming a closed circuit and containing a powder, part of the walls of said duct being of an electrically non-conductive material, means for inducing a flow of gas through said duct, electrical discharge means extending through said electrically nonconductive part of the walls of said duct for removing electric charge from said powder in said flow ofgas, capacitor means electrically connected to said electrical discharge means. and means separate from said duct for directing a flow of air containing the powder to be ejected adjacent to said capacitor means.

Since the powder in the duct in which charging takes place is separate from the powder which is to be deposited, the powder in the duct can be chosen to give optimum charging in conjunction with the material of which the duct is made. In addition, since no powder or air need be removed from the closed circuit, it can be completely sealed thereby reducing the risk of charge leakage and weak tribo charging, due to, say, humidity.

According to another aspect the present invention provides a device for the electrostatic deposition of particles such device comprising an apparatus for applying an electric charge to a powder comprising a duct having at least the inner surface of its walls formed of an electrically conductive material, means for causing a stream of gas containing said powder to flow through the duct, an earthed corona discharge electrode within said duct and means for applying a voltage to the inner surface of said duct Since. in accordance with the invention, the corona discharge electrode is earthed, that part of the corona current which goes to charge the powder is drawn from earth and only that part of the current which flows to the walls of the duct need pass through the voltage source.

Various embodiments of the invention will now be described by way of example and with reference to the accompanying drawings wherein:

FIG. 1 is a side view partly in section of an electrostatic spraying device in accordance with the invention;

FIG. 2 is a scrap side view of part of the device of FIG. I showing a container of powder fitted;

FIG. 3 is a diagrammatic view of a second portable electrostatic crop-spraying device;

FIG. 4 is a side view of part of a third portable electrostatic spraying device;

FIG. 5 is a schematic elevation of a fourth electrostatic spraying device; and

FIG. 6 is a schematic diagram of a means for imparting electric charge to a powder to be sprayed.

Referring initially to FIGS. 1 and 2 of the drawings the portable device illustrated comprises a short cylindrical discharge tube or barrel 1 approximately 15 inches long and 4 inches in diameter. In the embodiment illustrated the tube is manufactured from an electrically insulating plastics material. for example polymethylmethacrylate. The rear end of the barrel I is secured to the tangentially disposed outlet 2 ofa centrifugal fan 3. The fan has eight arcuate blades 4 disposed in a circular casing approximately l8 inches in diameter. A shaft 5 carrying the blades 4 is driven to rotate by a belt connected to an earthed electric motor 6. Extending axially from the centre of the fan casing is a cylindrical tube 7 (see FIG. 2). The tube 7 is cranked downwards at right angles and extends into the upper part of a container 8 for the powder 10 to be sprayed. The container 8 is of cylindrical form with the axis vertically disposed in use and has a perforated lid 9 which constitutes an air inlet. A cone with the point facing rearwardly may be mounted in the outlet of barrel 1 to reduce air motion at the target surface.

The projected rotational speed of the fan is approximately 10,000 rpm.

A toroidal capacitor electrode 11 formed of aluminium sheet surrounds the front end of the barrel I and a corona discharge needle I2 having a flat head [3 contacting the electrode 11 passes through the barrel wall and extends to a position centrally within the barrel.

In use, with the fan blades rotating, powder in admixture with air is sucked into the centre of the fan casing and is propelled through and out of the barrel in the direction of the crops or other articles to be sprayed. Contact between the blades 4 and the particles and also between the walls of the tube 7 and the particles imparts electro-static charge to each particle. This is the triboelectrical effect. The needle 12 creates a corona discharge through which electrical charge is conducted to the toroidal electrode 11 which in use becomes charged to a high electrostatic voltage, typically 200kv when adequately insulated, thereby by producing a directional electrostatic field extending towards the target from the front end of the barrel. The charged particles expelled from the barrel are guided by the electric field to the target (leaves of the plant or other object) to be sprayed which are at a relatively low potential for example earth. The particles thus adhere to the target thereby avoiding powder wastage.

It is sometimes found that the powder particles can be sufficiently charged within the fan by frictional contact with the fan blades. It has also surprisingly been discovered that electrostatic charging is eficctively achieved in the embodiment of FIGS. 1 and 2 if the particles are slightly moist. This is presumably because. on frictional contact, the moisture forms a conductive skin which conveys electrostatic charge to the entire periphery of the particle. With a damp particle, therefore, more or less sufficient electrostatic charging can be expected after one or a few contacts. In such circumstances furthcr contacts may lead to a partial discharge and this is the reason why in the device illustrated wherein charging takes place mainly in the fan a relatively short wide barrel is employed. In an alternative embodiment illustrated in FIG. 3 where charging is not sufficient in the fan and is intended to take place in an electro-conducting barrel a longer and narrower barrel lb is employed. Here the charging which takes place in the fan may be small compared to the charging achieved by contact with the walls of the barrel. Where charging of particles in the barrel is significant the Reynolds number of the two-phase air and particle flow along the barrel is significant. A Reynolds number greater than 5.000 is preferred. The barrel lh made of electrically conductive metal tubing is associated with a fan 3b, motor 6b and container (not shown) as in the em hodiment of FIG. 1. The means for charging toroidal electrode 1 1b in this embodiment may be a needle such as 13 in FIG. I conductivcly connected to the electrode 1111 by means not shown. Alternatively emerging charged particles may charge the electrode. Four radially extending undulating insulating spaces 50 extend between the electrode and the barrel.

The second electrostatic spraying device illustrated in FIG. 4 comprises a closed loop system. the front end part of which is shown. A connection to a fan inlet is made, preferably by a conducting or semiconducting tube connected by a circlip 14 to a length of insulating tube IS in turn connected to one end of an electroconductive metal e.g. aluminium or stainless steel tube U section 16 in an insulating outer sleeve I7. The other end of the U section I6 is connected to the fan outlet by a conductive metal tube 18. The front end part of the U section 16 has a discharge nozzle 19. A capping capacitor electrode 20 of generally spheroidal or toroidal configuration surrounds the U section 16 and serves for generating the guiding electrostatic field. The surface of electrode 20 is formed of electrically conductive material and connected by a needle 21 and wire 22 to the interior of the insulator tube l5. In use particles are fed into the loop system and are charged by triboelectricity. The charge is built-up by recirculation round the loop some of the particles being discharged out of the loop from the hole In.

Referring now to FIG. 5 an aluminium tube 23 is connected between a tube 24 made of an insulating material such as polymethylmethacrylate and one inlet 25 of a blower unit 26. The blower unit 26 comprises two centrifugal fans driven by the same motor but each having an air path which is completely separate from that of the other. The outlet 27 of the blower unit 26 which is associated with the inlet 25, is connected by a second aluminium tube 28 to the other end of the tube 24 so as to form a completely closed channel. The tube 24 is surrounded by a toroid 29 formed of electrically conductive material. A needle 30 of electrically conductive material, electrically connects the toroid 29 with the inside of tube 24.

The inlet 31 of the other part of the blower unit 26 is connected to a hopper 32 from which a mixture of air and the powder to be deposited can be fed into the blower 26. The corresponding outlet 33 is connected by a tube 34 to a nozzle 35, which directs powder into the region adjacent to the toroid 29.

The powder in the closed circuit formed by the tubes 23, 24 and 28 may. for example, be silicon carbide. In use. the blower unit 26 causes this powder to be circulated within the closed circuit. Triboelectric or contact- /separation charging takes place due to contact be tween the powder and the walls of the tubes 23 and 28. A corona discharge takes place at the needle 30 and the resulting charge is transferred to the outer surface of the toroid 29. Thus the toroid 29 is charged to a high voltage.

Meanwhile, powder from the hopper 32 is blown by the blower unit 26 along the tube 34 where triboelectric charging takes place. The material of which the tube 34 is made is chosen to be such that the polarity of the charge on the powder to be deposited is the same as that on the toroid 29. Thus, although the voltage on the particles of the powder emerging from the nozzle 35 may be relatively low. they are strongly repelled by the high voltage on the toroid 29 and deposited on any convenient earthed object in the vicinity, for example the object 36 which is the article to be coated. Thus, the closed circuit formed by the tubes 23. 24 and 28 performs the function of charging the toroid 29 to a high voltage to establish an electric field between the toroid 29 and an object 36 to be coated. At the same time. the powder to be deposited is charged in the tube 34.

Various modifications of the closed circuit part of the apparatus can be made. For example. a number of earthed needles may project into the tube 24. upstream and/or downstream of the needle 30. Baffles may be provided inside the tubes 23 and 28 to increase the turbulence of the gas flow therein. Instead of being made of aluminium, the tubes 23 and 28 may be formed of another metal or of a material having a soft inner surface and sufficient electrical conductivity to reduce the build up of stored charge on the tube. A suitable material is a resin-impregnated fabric having a thin rubber lining on its inner surface.

Alternatively. the whole of the tube forming the closed circuit may be made of an electrically non conductive material; a coaxial earth wire extending the full length of the tubes 23 and 28 to remove surface charge from the walls thereof.

As an alternative, or in addition to triboelectric charging, the particles both in the closed circuitand in the tube 34 may be charged by corona discharge. For this purpose, one or more corona discharge electrodes extend into the charging region of the tube. Such electrodes are electrically insulated from the adjacent walls of the tube and a potential difference of sufficiently high voltage to establish a discharge is applied between the corona discharge electrode and the adjacent walls of the tube. With conventional sharp-edge electrodes a voltage of about I to 20 kV would be sufficient.

Referring now to FIG. 6 of the drawing, a tube 37 of electrically conductive material such as aluminium, is connected to form a closed circuit with two tubes 38 and 39 of electrically insulating material and a fan 40. The tubes 37, 38 and 39 contain powder so that. when the fan 40 is in operation. such powder is blown round the closed circuit.

Within the tube 37. a coaxial electrically conductive wire 41 is stretched between end supports 42 and 43 of insulating material. The wire 41 is electrically con nected to a second wire 44 which passes through an in sulator 45 in the wall of the tube 37 and has its outer end connected to earth. The wall of the tube 37 is connected to the output of a generator 46 which produces an output voltage in the range 5 to 25 kV.

The tube 39 which, it will be recalled, is of electrically insulating material, is surrounded by a toroid 47 of electrically conductive material. A needle 48 of electrically conductive material, projects from within the toroid 47, to which it is electrically connected, into the tube 39. The fan 40 is arranged to circulate powder within the tube in a clockwise direction as viewed in the drawing so that the needle 48 is downstream of the tube 37.

In use, with the generator 46 in operation, the fan 40 circulates powder within the tubes 37, 38 and 39. A corona discharge is established from the wire 4!, drawing current from earth through the wire 44. Part of this current goes to charge the powder circulating in the tube and the remainder flows through the walls of the tube 37 and the generator 46. For example with a powder flow rate of. say, kg per hour, as much as 75% of the current flowing from earth through the wire may go to charge the powder depending on the particle size and shape. Thus, if the generator 46 is capable of supplying a current of p.A, the total current flowing through the wire 42 can be as high as IOO A. With prior art apparatus, the total current would have been limited to the current which could have been supplied by the gen erator 46.

As the charged powder circulates into the tube 39, a corona discharge is established at the needle 48 so that charge is transferred from the powder to the toroid 47.

This charge transfers to the outer surface of the toroid 28 which thus rises to a very high voltage of the order of 250 kV. with improved insulation, this voltage could be even higher.

In order to prevent the risk of corona discharges being established at any sharp edges which may occur at the ends of the tube 10, the tubes 12 and 14 are formed with spigots 49 and 50 respectively which cover the inner surface of the ends of the tube 7.

A significant amount of triboelectric charging of the powder can take place. The materials of which the tubes 37, 38 and 39 are made are preferably chosen to be such that, in conjunction with the powder used, the polarity of the charge on the powder due to triboelectric charging is the same as that due to corona charging. Also, the Reynolds number should be kept low.

In an electrostatic powder deposition apparatus the powder with which an object is to be coated, is blown through a tube generally similar to the tube 37 but provided with a nozzle.

I claim:

1. A device for electrostatically depositing particles onto a surface at earth potential, such device comprising a casing, means for introducing particles to be de posited into the interior of the casing, a discharge barrel extending from the casing and triboelectricity inducing means movable within the casing which also propel particles from the casing through and out of the barrel.

2. The device of claim 1 wherein the triboelectricity inducing means comprises a fan.

3. The device of claim 2 wherein the casing is generally circular in form, wherein a connection from a container for the particles extends to the centre of the easing and wherein a shaft carrying the fan blades extends along the said centre.

4. The device ofclaim 2 wherein the barrel is tangentially disposed to the casing, the fan being a centrifugal fan with arcuate blades.

5. The device ofelaim 1 comprising means for generating an electrostatic field extending from the front of the barrel.

6. The device of claim 5 wherein said means are constituted by an electrode surrounding the front of the barrel and means for charging the said electrode from charged particles in the barrel.

7. The device of claim 5 wherein said means concentrate a cloud of particles emerging from the nozzle so as to create a high-voltage space-charge to act as a directional electric field.

8. The device of claim 6 wherein the said means of electrode charging is a corona discharge needle.

9. The device of claim 6 wherein the said means of electrode charging is an electrical conductor positioned to contact charged particles.

10. The device of claim 6 wherein the electrode is to roidal or spheroidal.

11. The device of claim 1 including moist particles in the interior of the casing.

12. A device for electrostatically depositing particles on a surface at a lower potential such device comprising a tubular conduit, means for introducing particles into the conduit, means for driving particles along the conduit to a discharge outlet, means for inducing by triboelectricity electric charge to the particles driven along the conduit, electrically chargeable means for generating a directional electric field around said dis charge outlet and means for conduetively connecting said electrically chargeable means with charged particles in the conduit.

13. The device of claim 12 including means of concentrating the charged cloud at the nozzle so as to produce a directional electrical field by the spacecharge potential.

14. The device of claim 12 wherein the conduit forms a loop connecting the inlet and the outlet of a fan.

15. A device for electrostatic-ally depositing particles such device comprising a first duct forming a closed circuit and containing a powder, part of the walls of said duct being of an electrically non-conductive material, means for inducing a flow of gas through said duct, electrical discharge means extending through said electrically non-conductive part of the walls of said duct for removing electric charge from said powder in said flow of gas, capacitor means electrically connected to said electrical discharge means, and means separate from said duct for directing a flow of air containing the powder to be ejected adjacent to said capacitor means,

16. A device for the electrostatic deposition of particles such device comprising an apparatus for applying an electric charge to a powder comprising a duct having at least the inner surface of its walls formed of an electrically conductive material, means for causing a stream of gas containing said powder to flow through the duct, an earthed corona discharge electrode within said duct and means for applying a voltage to the inner surface of said duct. 

1. A device for electrostatically depositing particles onto a surface at earth potential, such device comprising a casing, means for introducing particles to be deposited into the interior of the casing, a discharge barrel extending from the casing and triboelectricity inducing means movable within the casing which also propel particles from the casing through and out of the barrel.
 2. The device of claim 1 wherein the triboelectricity inducing means comprises a fan.
 3. The device of claim 2 wherein the casing is generally circular in form, wherein a connection from a container for the particles extends to the centre of the casing and wherein a shaft carrying the fan blades extends along the said centre.
 4. The device of claim 2 wherein the barrel is tangentially disposed to the casing, the fan being a centrifugal fan with arcuate blades.
 5. The device of claim 1 comprising means for generating an electrostatic field extending from the front of the barrel.
 6. The device of claim 5 wherein said means are constituted by an electrode surrounding the front of the barrel and means for charging the said electrode from charged particles in the barrel.
 7. The device of claim 5 wherein said means concentrate a cloud of particles emerging from the nozzle so as to create a high-voltage space-charge to act as a directional electric field.
 8. The device of claim 6 wherein the said means of electrode charging is a corona discharge needle.
 9. The device of claim 6 wherein the said means of electrode charging is an electrical conductor positioned to contact charged particles.
 10. The device of claim 6 wherein the electrode is toroidal or spheroidal.
 11. The device of claim 1 including moist particles in the interior of the casing.
 12. A device for electrostatically depositing particles on a surface at a lower potential such device comprising a tubular conduit, means for introducing particles into the conduit, means for driving particles along the conduit to a discharge outlet, means for inducing by triboelectricity electric charge to the particles driven along the conduit, electrically chargeable means for generating a directional electric field around said discharge outlet and means for conductively connecting said electRically chargeable means with charged particles in the conduit.
 13. The device of claim 12 including means of concentrating the charged cloud at the nozzle so as to produce a directional electrical field by the spacecharge potential.
 14. The device of claim 12 wherein the conduit forms a loop connecting the inlet and the outlet of a fan.
 15. A device for electrostatically depositing particles such device comprising a first duct forming a closed circuit and containing a powder, part of the walls of said duct being of an electrically non-conductive material, means for inducing a flow of gas through said duct, electrical discharge means extending through said electrically non-conductive part of the walls of said duct for removing electric charge from said powder in said flow of gas, capacitor means electrically connected to said electrical discharge means, and means separate from said duct for directing a flow of air containing the powder to be ejected adjacent to said capacitor means.
 16. A device for the electrostatic deposition of particles such device comprising an apparatus for applying an electric charge to a powder comprising a duct having at least the inner surface of its walls formed of an electrically conductive material, means for causing a stream of gas containing said powder to flow through the duct, an earthed corona discharge electrode within said duct and means for applying a voltage to the inner surface of said duct. 